Buried or immersed metal structures, such as pipes and tanks, are exposed to an electrochemical corrosion process in the underground environment. The metal structure becomes an electrode and the soil an electrolyte so that an electrolytic cell is formed causing the corrosion of the buried structure. Some corrosion arises from naturally occurring processes at specific locations on the underground structure involving electrical current flow into the ambient soil electrolyte from sites acting as anodes via the corrosion reaction. Current will flow to specific locations on underground structures acting as cathodes where reduction reactions occur. Corrosion can also be caused or accelerated by voltages applied to a local region of the pipe by manmade structures, including local transit systems, overhead sub-surface power distribution systems, and industrial plants.
There are generally two methods to protect the underground structure from corroding. These methods are referred to as “cathodic protection.” The first method is to attach sacrificial anodes onto or near the underground structure being protected, and the second is to install an impressed current system that generates a DC voltage and a low constant current in the specific vicinity of the structure being protected.
Sacrificial anodes are pieces of metal more electrically active than the underground structure that can be attached to the underground structure for corrosion protection. Because these anodes are more active than the underground structure, the corrosive current will exit from the sacrificial anodes rather than from the underground structure. Thus, the underground structure is protected while the attached anode is sacrificed. Depleted anodes must be replaced for continued corrosion protection of the underground structure.
An impressed current system uses a rectifier that converts alternating current to direct current. This current is sent through an insulated wire to the anodes, which are metal bars buried in the soil near the underground structure being protected. The current flows through the soil to the underground structure and returns to the rectifier through an insulated wire attached to the underground structure. The underground structure is protected because the current going to the underground structure overcomes the corrosion-causing current normally flowing away from it.
With a cathodic protection system, industry accepted criteria involve the measurement of the electrochemical potential of the structure to establish the level of cathodic protection sufficient to mitigate corrosion of the buried metal structure. The ordinary practice to determine the necessary level of cathodic protection is to measure the potential difference between the buried structure, which is an electrode, and a reference electrode placed, at grade, in contact with the soil, which is an electrolyte. However, when this measurement is taken while the cathodic protection system is operating, a voltage drop through the soil due to the cathodic protection current, referred to as the IR (voltage) drop, causes an error in the potential measurement.
In order to measure a potential that is free of IR drop, it is common to measure the potential immediately following interruption of the cathodic protection current. The instantaneous voltage drop which occurs immediately after the cathodic protection is turned off is equal to the IR drop caused by the interrupted cathodic protection current. Because the electrochemical interface of the protected structure has a capacitive component, the potential of that interface does not change immediately following interruption as does the IR drop. Therefore, the potential measured immediately following interruption of the cathodic protection current, when current, I, is zero, is the potential of the protected structure free of IR drop. This potential is referred to as off-potential.
Problems arise in interrupting the cathodic protection. Extremely long buried pipelines have multiple cathodic protection stations, all of which must be interrupted simultaneously, or interrupted using a non-synchronous method in which all of the IR drops are summed. Galvanic cathodic protection systems are not designed to be interrupted because the anodes are typically directly connected to the protected structure. Additionally, second-party cathodic protection systems that are either unknown or cannot be interrupted may be present in the area. Other problems exist, also, such as stray current effects, e.g., from power distribution systems, dc transit systems, etc., that are not interruptable, and rapid IR transients, which immediately follow interruption. For instance, AC current magnetic fields created in the vicinity of high voltage power lines can propagate into the earth and induce corrosion. This is known as AC-induced corrosion. Such corrosion on cathodically protected underground pipelines is commonly the result of a combined action of the induced AC voltage, the cathodic protection conditions, a piping coating defect and the chemical and physical conditions of the soil. As a result, both DC and AC current fields can influence corrosion of underground structures.
In order to avoid the problems associated with interruption of the entire cathodic protection system, “coupons” are used to monitor the level of cathodic protection on buried metal structures such as pipes. The coupon is a bare metal sample having substantially the same metallurgical attributes as the pipe or other structure being monitored. The coupon is placed in the soil near the pipe and connected to the pipe. Therefore, the coupon is exposed to the same cathodic protection current source as the pipe. The connection between the pipe and the coupon is interrupted for a time period, during which time the potential difference between the coupon and a reference electrode is measured. The pipe's cathodic protection is never interrupted, since only the pipe-to-coupon connection is interrupted. The coupon's potential simulates the potential of a pipe coating defect of a similar surface area as the coupon. The coupon's potential can be measured without interrupting the cathodic protection to the pipe, and therefore without some of the problems inherent in interrupting the entire cathodic protection system. In addition, once the coupon is interrupted, it is an isolated, small piece of metal in the soil and stray currents are eliminated from its surface. In contrast, stray currents are generally not eliminated from a pipeline upon interruption of the entire cathodic protection system.
Despite the existence of a significant number of devices for determining the effectiveness of cathodic protection, the need exists for a simple, inexpensive and accurate measuring device. In the description provided above, the coupon serves as a simulation of the behavior of the underground structure that is being protected. The coupon is used for measuring changes in electrical potential near and in the vicinity of the underground structure that is being protected. The changes in electrical potential are characteristically “noisy.” That is, the changes in the electrical potentials or the resistance of a coupon are not always consistent and are affected by temperature variations, the presence of stray currents themselves, instrumentation calibration challenges and operator error. There is a need for coupons that are not impacted by environmental or operator data-acquisition variables; coupons that can be measured with more precision and accuracy.